An impression occurs when an advertisement is placed on a web page rendered for display on a client device such as a desktop computer or a mobile device (e.g., multimedia phone). Depending on the web page and the client device, the advertisement may not be visible to a user of the device because the advertisement is placed on a portion of the web page that is not in the device's viewport. For example, because of the zoom level of the viewport (e.g., the viewport is zoomed-in), the size of the viewport and/or the dimensions of the web page, not all of the web page's contents can be displayed in the viewport at the same time. For mobile devices displaying a web page such a scenario is a common occurrence because of the mobile device's relatively small viewport. Because advertisers are typically charged for an impression if the advertisement is placed on a rendered web page, an advertiser may be charged even if the advertisement is placed on a portion of the web page that is not in the viewport and, thus, not visible to the user.
Further, depending on the web page and the client device, the web page may be displayed in a viewport at a zoom level that permits the entire page to be displayed in the viewport (e.g., the viewport is zoomed-out). Many mobile device's employ this strategy to present the entire web page to the user. However, typically, at such zoom levels the contents of the web page are illegible or difficult to discern, including any advertisements. Thus even though the zoom level of the viewport may prevent an advertisement from being read by a user, the advertiser may nonetheless be charged for an impression, as the advertisement was placed on a rendered web page.